This invention relates to the coating of air bags, which are used for safety purposes to protect occupants of vehicles such as automobiles, and of air bag fabrics intended to be made into air bags. In particular the invention relates to the top coating with an anti-blocking composition of air bags and air bag fabrics which have been coated with a composition comprising an organic resin. By an ‘organic resin’ or ‘organic polymer’ we mean a polymer in which at least 50% of the atoms forming the polymer chain are carbon atoms.
Air bags are generally formed from a woven or knitted fabric made of synthetic fibre, for example of polyamide such as nylon-6,6 or polyester, covered on at least one of its sides with a layer of an elastomer. Air bags may be made of flat fabric pieces which are coated and then sewn together to provide sufficient mechanical strength, or may be woven in one piece with integrally woven seams. Sewn air bags are generally assembled with the coated fabric surface at the inside of the air bag. One piece woven air bags are coated on the outside of the air bag.
For some airbag applications, pressurised gas has to be retained in a fabric envelope for a relatively long period. This requirement exists for example in side curtain airbags for the automotive industry. These side curtain airbags are intended to inflate at the time of impact, as do conventional airbags. The side curtains unfold to form a cushioned curtain between passengers and some of the side of the car body, e.g., the windows. As the intention is not merely to cushion the blow on impact itself, as is the case for conventional driver and passenger airbags, but to protect passengers e.g. when a car is rolling, it is important that the side curtain air bag is sufficiently pressurised during such rolling process. Where conventional driver and passenger airbags only need to retain pressure for a fraction of a second, it is desirable that side curtain airbags maintain a suitable pressure for a few seconds. Similar applications exist where a pressurised fabric structure is desired to maintain a certain gas pressure for a relatively extended period of time, e.g. in emergency chutes for aeroplanes, or inflatable rafts. There is thus a demand for coated fabrics having the benefits of flexibility and high temperature resistance at low coating weight given by silicone rubber coatings, but with improved air tightness.
Among preferred elastomers for coating the air bag or air bag fabric are urethane polymers.
U.S. Pat. No. 5,110,666 describes a fabric substrate which is coated with a novel polycarbonate-polyether polyurethane for use as a driver's side or passenger side air bag.
U.S. Pat. No. 6,169,043 describes airbag coating compositions comprising a mix of polyurethane and polyacrylate constituents to provide a low permeability coating on a fabric surface.
U.S. Pat. No. 7,543,843 describes the use of hybrid resins as airbag coatings. The hybrid resins are urethanes blended with acrylates, vinyls, and/or silicones, where at least one of the components has a glass transition temperature of 20° C. or less. The urethanes are preferably of the polycarbonate, polytetramethyleneglycol, silicon-based diol, or olefin-based diol type.
If a urethane polymer base coat is left as the only coating on the air bag, the surface properties of this base coat would result in blocking (the urethane polymer coated surfaces sticking to each other during storage and tight packing of the air bag in the automobile, particularly at high ambient temperatures) and very high stresses when the airbag is inflated which would result in bag failure by tearing during inflation or by delamination of the urethane polymer base coat from the fabric. Blocking between urethane polymer surfaces is also a problem during manufacture of air bags when fabric coated with urethane polymer is stored in a roll.
U.S. Pat. No. 5,945,185 describes an air bag made of silicone modified thermoplastic polyurethane resin in which the content of siloxane is 5-40% by weight. Such an air bag is claimed to have reduced danger of blocking, but vehicle manufacturers have preferred to use coated fabric air bags.
U.S. Pat. No. 6,239,046 describes coating a knit, woven, or non-woven textile substrate with an adhesive polyurethane layer and then with an elastomeric polysiloxane layer. An air curtain or air bag with superior air-holding and superior heat resistance is then formed from the coated textile substrate.
U.S. Pat. Nos. 6,177,365 and 6,177,366 describe airbag coatings comprising at least two separate layers. The first layer (base coat), in contact with the airbag surface, comprises a non-silicone composition of polyurethane, polyacrylate, polyamide, butyl rubber, hydrogenated nitrile rubber or ethylene vinyl acetate copolymer. The second layer (topcoat) is a silicone material.
Moreover, it has been found that the application of a curable liquid silicone rubber top coat over certain organic resin base coats, particularly urethane polymer base coats or base coats cured with an amino resin, releases a displeasing fishy smell.